1. Field of the Invention
The present invention relates to a strain measuring device able to switch easily the ranges of amount of the strain and which can eliminate the influence of drift voltage produced in a driver circuit of the bridge current source of the strain measuring device using a Wheatstone bridge circuit, and can also eliminate the influence of the thermal electromotive force induced inside or outside the Wheatstone bridge circuit.
2. General Description of the Prior Art
In a conventional strain measuring device using a Wheatstone bridge circuit (hereinafter abridged and referred to as a bridge circuit) an accurate measuring value cannot be obtained when a thermal electromotive force is induced at connections inside or outside of the bridge circuit, because such thermal electromotive force may be applied to the measuring system besides the desired unbalanced voltage, which is in proportion to the amount of the strain to be measured.
Various devices are hitherto known to eliminate such adverse influence of the thermal electromotive force. For instance, a strain measuring device is known in which the polarity of the bridge current source voltage for exciting the bridge circuit is reversed in two successive measurements, and the obtained measuring values are subtracted one from another and divided by two to eliminate the thermal electromotive force. Further it is known to use a measuring device, in which a first measurement is made under the impression of an exciting voltage to the bridge circuit and then a second measurement is made at a condition such that the exciting voltage is interrupted, and the thus-obtained second measurement is subtracted from said first measurement to eliminate the influence of the thermal electromotive force.
In order to provide a clear understanding of the invention, such prior art devices will be explained in more detail below.